This invention relates to large land grid array integrated circuit packages, and in particular to removal of heat from integrated circuit chips mounted on such packages through the use of heat pipes.
Packaging of semiconductor chips is a key contributor to the cost, performance, and reliability of any electronic system, especially electronic digital computer systems. Early transistorized computer systems, such as the IBM 1401, had individual transistors mounted in metal or plastic units with the transistor""s three wires extending from the package. These wires would typically be inserted through holes in a printed wiring board (PWB) that had printed wiring on a top or bottom surface, or in some instances, on one or more internal planes in the card. A soldering operation would then electrically connect the transistor""s three wires to the printed wiring on the PWB. Resistors, capacitors, diodes, and other electronic components would also be connected in the same way on the PWB to complete the desired circuitry, e.g., logic NANDs, logic NORs, latches, and so forth. The wiring length between these components reduced performance due to time of flight delays. Lengthy wires between the transistors and the PWB also introduced inductive and capacitive parasitics that degraded signal integrity.
By the mid 1960s, multiple transistors and resistors were placed in a single electronic package, or module. Modules were then connected physically and electrically to the PWB with conductors called pins that carried signals from the circuitry on the module to the PWB. These pins would go through holes in the PWB and solder would then make electrical and mechanical connections. These pins could be arranged along the edges of the module or could be arranged as an array of pins protruding from the bottom of the module. These pins brought power and ground connections to the circuitry on the module from the PWB and also provided paths for logic signals between the circuitry and the PWB. These pins were still very limiting in terms of providing low inductance power and ground to the circuitry and in the number of signals that could be received or driven between the circuitry on the module and the PWB.
Still later, direct solder attach between the module and the PWB was accomplished through xe2x80x9cball grid arrayxe2x80x9d, xe2x80x9csolder bumpxe2x80x9d, or xe2x80x9csolder columnxe2x80x9d packages. Although these packages were electrically superior to the wired or pinned packages they had some limitations. One limitation was the difficulty of reworking the PWB to remove and replace a faulty module. Another limitation was the allowable width and length of the module. This limitation resulted from coefficient of thermal expansion (CTE) mismatches between the materials in the module and the materials in the PWB. The solder connections mechanically fatigued when flexed repeatedly. During a temperature change the solder connections flexed in proportion to how far they were from a central, or neutral, point on the module. Therefore, solder connections on larger modules became fatigued and electrically unreliable from normal thermal variations when the computer was turned on and off repeatedly, or even during normal periods of greater or lesser logic switching activity during normal use. Limitations of this kind of module to PWB connection restricted such packages to moderate width and length dimensions and number of rework cycles that could be performed.
A recent addition to packaging technology is the Land Grid Array (LGA) module. These LGA modules are socketed directly to the PWB with socket connectors, in which the electrical connection is achieved by aligning a contact array on the bottom of the module, a contact array on the top surface of the PWB, and an LGA interposer. The LGA interposer is then compressed between the module and the PWB. Upon sufficient compression, the LGA interposer provides electrical connection between the respective module and PWB array points. The advantage of LGA socket connectors include: field upgradeability; flexibility in system bring-up and diagnosis; reduced PWB assembly rework cost; reduced effects of CTE mismatch between PWB and module; improved electrical performance; and compact designs. There are various types of LGA interposers, including a version based on finely twisted wire, a technology marketed by Cinch Connectors Company. In this approach, palladium or gold plated molybdenum wire is randomly wound to form individual contacts, somewhat similar to a miniature wound-wire scouring pad. These contacts are housed in plastic molded frames. A similar xe2x80x9cscouring padxe2x80x9d connector is made by Tecknit Corporation. Another type of LGA interposer is a Metallized Particle Interconnect (MPI(trademark)) technology such as is marketed by Tyco International, Ltd. MPI(trademark) has silver-filled elastomeric contacts which are molded onto and through a carrier such as Kapton(copyright), manufactured by the DuPont Corporation, which is staked onto a plastic frame. Yet another type of LGA interposer is the cLGA(trademark) product from InterCon Corporation in which C-shaped clips are individually inserted into cavities molded in a plastic form. Further examples of interposer technologies can be found in the literature and US patents.
LGA socket assemblies are common today in the electronics industry but are typically used to attach single chip modules (SCMs) to PWBs. Relatively small multichip modules (MCM) also exist in the art. The demand for system performance is driving a requirement to place a number of chips, amounting to a large silicon area, on an MCM, while at the same time requiring a very large number of signal, ground, and power supply connections. Each electrical connection on an LGA interposer requires a small amount of force. However in today""s large and very high speed systems there are enough connections that well over a half ton of force is required to compress the interposer enough to make reliable connections. Torqueing screws at the corners of the modules has accomplished conventional compression of the LGA interposer. With the increasing size of the sockets and modules, and the large forces required to compress the interposer, it has become problematic to keep the module and the card from unacceptably deforming when tightened from the corners in the conventional manner. Conventional compression pinches the edges of the module against the PWB underneath the edges with no concentrated force under the module itself, which means that the individual LGA connections towards the edge are compressed or deformed with more force than those LGA connections near the middle. Such differences in deformations can cause unreliable connections on the LGA interposer.
Another vital concern in the design of packaging of Very Large Scale Integrated (VLSI) circuit chips is the removal of heat that is produced from the circuitry on the chips. Chip temperature must not be allowed to exceed some maximum temperature because of reliability and performance considerations. Many designs in the past have provided paths for thermal conduction to fins for air cooling or to portions of the module that are cooled by liquids, such as water. A cooling mechanism that has recently been introduced to the field of cooling chips is the heat pipe. Heat pipes have superior heat transfer characteristics but tend to be physically large and must be pressed against the chip with a predefined force. Current use of heat pipes to cool chips involve torqueing tools or methods of counting turns of a screw to ensure proper loading of the heat pipe upon the chip. These methods of loading are error prone, time consuming, and expensive. Furthermore, prior technologies did not support large, LGA MCMs with heat pipe cooling of the chips.
Therefore, there is a need for an LGA socket assembly capable of holding a large MCM and compressing one of a variety of commercially available LGA interposers between the MCM bottom and the PWB top in a manner that maintains substantially uniform force across the LGA interposer. Such an LGA socket assembly further must provide for excellent heat transport away from the chips through the use of heat pipes and the heat pipes should be loaded against the chips without the requirement of counting of screw turns or torque measurement.
This invention provides an apparatus that provides for substantially uniform loading of an LGA interposer between an MCM and a PWB. The apparatus further provides a mechanism for loading of heat pipe cooling devices against semiconductor chips on the MCM without the requirement of torqueing tools or counting of turns of a screw.
The substantially uniform loading of the LGA interposer is accomplished by turning an actuation screw, which forces down upon the top of a module cap on the MCM. The screw is threaded through an X-shaped loading mechanism. The loading mechanism is mechanically coupled to load posts, which extend through or past the MCM cap; through or past an LGA interposer; through the PWB; through or past a backside stiffener; and are mechanically joined to a load spring. As the actuation screw is actuated, the loading mechanism is forced away from the module cap, putting tension on the load posts and transferring the force to the load spring. A load bushing situated substantially at the center of the load spring pushes against a backside stiffener at or near the center of the backside stiffener, which in turn, pushes against the PWB. This produces a slight convex deformation of the backside stiffener and the portion of the PWB the backside stiffener pushes against. The PWB in turn pushes upward, compressing the LGA interposer against the MCM. Although the actuation screw pushes down at the center of the MCM cap, the module cap supports the MCM at the edges. As the LGA interposer is pressed against the MCM, the center of the MCM is also slightly deformed in a convex manner in the same direction as the backside stiffener, the PWB, and the LGA interposer. In this manner, force can be distributed substantially evenly across the many contacts between the bottom of the MCM and the top of the LGA interposer and also between the bottom of the LGA interposer and the top of the PWB.
Power dissipated on the semiconductor chips must be removed from the chips. Very high-speed processor chips currently dissipate well over 100 watts. The chips must be kept below some maximum temperature for performance and reliability considerations. Current semiconductor chips used in commercial computers typically have a maximum temperature of about 100 degrees centigrade specified. Reliability of the semiconductor chips degrades rapidly beyond this temperature, forcing designers to reduce current densities in wiring on the chip. Other reliability considerations force other design limitations as temperatures rise. Complementary Metal Oxide Semiconductor (CMOS) circuits also degrade in performance as temperature rises, providing even further motivation to effectively remove heat from the chips.
Heat pipes are an excellent means to remove heat. This invention provides a mechanism to load a heat pipe against a chip with a predetermined amount of force without requiring use of torqueing tools or counting turns of a screw. A loading force of the heat pipe against the chip in this invention is determined by the geometries of a load collar and a piston, together with the geometry and spring constant of a spring. The load collar and the spring are coaxially and slideably placed on the heat pipe. The load collar is simply forced downwards, through a hole on the MCM cap, until a flange on the load collar comes into contact with the top of the MCM cap, which stops further travel of the load collar. The spring is compressed between a bottom of the load collar and a top of a piston affixed to a bottom end of the heat pipe, forcing a bottom of the piston against a top of the chip with a force determined by the geometries and the spring as stated above. O-rings on the inner and outer peripheries of the load collar prevent contamination from entering the volume between the top of the MCM and the MCM cap.